Structured liquid surfactant personal care compositions are desirable. Such structured liquids can be used, for example, to suspend beads and/or other particles desirable in personal care compositions. Such particles can be used, for example, as abrasives, encapsulates (e.g., for delivering additional benefit agents), or to provide visual cues (e.g., optical particles).
Typically, particles may be suspended in liquid personal care compositions using a variety of structuring systems. These may include use of acrylate polymers, structuring gums (e.g., xanthan gum), starch, agar, hydroxyl alkyl cellulose etc. When large particles are suspended (e.g., polyethylene particles, guar beads), levels of polymer used is typically 1% or more.
It has previously been shown that when certain fibrous polymers (e.g., micro fibrous cellulose with large aspect ratios) are used as structurants, these may provide efficient suspending properties even at polymer levels as low as 0.1% (see U.S. Pat. No. 7,776,807 to Canto et al.; U.S. Publication No. 2008/0108541 to Swazey and U.S. Publication No. 2008/0146485 to Swazey). The fibrous polymers are believed to form spider-network like structures which efficiently trap the particles inside the network and thereby impart good suspending properties. The polymers provide excellent rheological properties and are salt tolerant, if salt is used in the formulation. The microfibrous cellulose (MFC) polymers used, however, have a zero or near zero stress-shear rate profile (i.e., zero stress-shear rate slope when plotting shear rate versus stress). One problem associated with zero stress shear rate slope is flow instability and MFC alone will not eliminate this problem. One of the goals of the subject invention is to eliminate such zero stress-shear rate slope, thereby resolving the problem of flow instability.
Fibrous polymer, such as micro fibrous cellulose, has been used, for example, in liquid laundry detergent compositions. WO 2009/135765 (Unilever), for example, discloses a process for making structured liquid detergent composition comprising micro fibrous cellulose. The compositions comprise 25-55% surfactant (we use 15% or less, preferably, 10% or less by wt. in our compositions). There is no disclosure of flow instability (causing product lumpiness, also known as shear banding) or of use of specific polymers to be used in conjunction with MFC to resolve such issue.
WO 2009/101545 (P&G) also discloses liquid detergent compositions comprising micro fibrous cellulose. The reference also discloses typically much higher amounts of surfactant than used in our compositions. Further, compositions of our invention comprise neither enzymes nor chelators/builders, typical ingredients found in laundry detergent compositions.
U.S. 2007/0197779 (CP Kelco) discloses structurant consisting of bacterially product MFC with carboxymethylcellulose and xanthan gum as dispersion aids. Practical difficulties arise when this type of thickener is used with surfactant containing compositions. Microfibrous cellulose, as noted above, will not by itself eliminate the problem of flow instability (associated with zero or near zero stress shear rate slope) in surfactant structured compositions. The compositions are also enzyme-containing detergent liquids.
U.S. Pat. No. 7,776,807 (noted above) discloses liquid cleansing compositions comprising micro fibrous cellulose (MFC). As indicated, rheological properties of the composition include high zero or near zero stress-shear rate slope (associated with flow instability which in turn causes shear banding). Again, fibrous polymers alone do not eliminate zero or near zero rate slope which are particularly a problem in surfactant structured liquid compositions. In such compositions, use of salt to enhance viscosity can result in flow instability and product lumpiness. Applicants seek to protect against flow instability in viscosity ranges from 100 cps to 100,000 cps, preferably 500 to 50,000 cps.
Flow instability or “shear-banding”, compared to shear thinning is disclosed generally in “Comparison between Shear Banding and Shear Thinning in Entangled Micellar Solutions”, Hu et al., J. Rheol., 2008, 52(2), 379-400; and “Role of Electrostatic Interactions in Shear Banding of Entangled DNA Solutions”, Hu et al., Micromolecules, 2008, 41, 6618-6620. Examples A-E of a surfactant system structured with entangled wormlike micelles with and without fibrous polymers, exhibits the undesired flow instability or shear-banding behavior.
Specifically, the phenomenon can be readily observed from compositions having zero or near zero stress-shear rate slope profile measured by standard rheological measurements. FIG. 1 shows typical flow profiles displaying zero stress-shear rate slope for Examples A, C and E. The details of the measurement and instrument are given in the protocol.
From this Figure, it can be seen that there is a zero or near zero slope, for example, in range of 10 to 1000 s−1 (shear rate). If stress or force is applied to a liquid composition over this range, a zero-slope curve implies that a single force can have multiple shear rates or flow rates. This is what is meant by “flow instability” and it is such flow instability which causes lumpiness or shear banding.